8.1 Setting variables

A variable definition is recognized by the equals sign in the first field of the input card.
For example,

THRESH,ENERGY=1.d-8,GRADIENT=1.d-5

does not define variables; here ENERGY and GRADIENT are options for the THRESH
directive.

Variables can have different types:

Numbers:

The value is a number or an expression. The general form of value is
expression [,] [unit]
unit is an optional string which can be used to associate a unit to the value. ANG[STROM], DEGREE,
HARTREE are examples. Undefined variables in expressions are assumed to be zero (and defined to be zero at the same time).

Logicals:

The value can be .TRUE. or .FALSE. (.T. and .F. also work), or a logical expression.
Internally, .TRUE. is stored as 1 and .FALSE. as zero.

Strings:

The value can either be a string enclosed in quotes or a string variable.
See section 8.3 for more details.